Category: Conformations and Cycloalkanes

In the previous lesson we started our discussion of structures with more than one ring, using decalin as our key example of a fused ring. We saw how much the stereochemistry at the ring junction can affect the overall shape of the molecule, as well as its stability. 1. Fused Rings, Bridged Bicyclic Rings, And Spiro Rings What we didn’t talk…

At the beginning of this series I said that the fact that carbon can form rings leads to all kinds of interesting consequences. We’re going to see many examples of that in our lesson today! So far, we’ve only talked about cyclic molecules containing one ring. But, of course, molecules with multiple rings are very common…

Finding The Most Stable Conformation Of A Cyclohexane Chair You’re given a structure with two or more substituents on a cyclohexane ring, and you’re asked to draw the most stable conformation. How do you do that? That’s what this lesson is about. 1. A-Values Are A Useful Measure Of Bulkiness In the last lesson, we introduced A…

“A-Values” For Substituted Cyclohexanes “A-Values” are a numerical way of rating the bulkiness of substituents on a cyclohexane ring. The “A-Value” represents the difference in energy (in kcal/mol) between the cyclohexane conformation bearing the group in the equatorial position (more favored) and the cyclohexane conformation bearing the group in the axial position.  The greater the…

Equatorial vs Axial Groups: Why The Equatorial Position Is Of Lower Energy Just to bring you up to speed,  let’s quickly review the last lesson.  And at the bottom, I’ll also correct a little fib I made in the last lesson. 1. Brief Review On The Cyclohexane Chair Conformation Now, the correction to the fib. In the…

Energy Diagram Of The Cyclohexane Chair Flip In the last lesson,  we showed a video of  a cyclohexane ring flip – turning a cyclohexane chair conformation into a boat and then into the opposite chair. The key observation we made here was that a chair flip converts all axial groups into equatorial groups and all equatorial groups into…

1. The Two Chair Conformations Of 1-Methyl Cyclohexane In a recent lesson, an aerial tour of the cyclohexane chair, we showed that there are two different positions a substituent can occupy on a cyclohexane chair – axial (straight up and down, relative to the ring) and equatorial (off to the side of the ring). This brings…

How To Easily Draw Your First Cyclohexane Chair Using the “Parallelogram Technique” Now that we’ve had an aerial tour of the cyclohexane chair, we’re going to move on to a very important skill: how to properly draw one of these beasties. There are several techniques for properly draw a cyclohexane chair. In this video, I walk through what…

A Fly-by Of The Cyclohexane Chair Conformation Sometimes words just don’t do the job, so I built a model of the cyclohexane chair and took a whole bunch of videos and pictures of it to help you get the picture. 2. It’s Only A Hexagon If You Look At It From the Top When I…

Conformations of Cyclohexane (and Cyclopentane) In the last lesson, we saw that ring strain of cyclopropane and cyclobutane were 27 and 26 kcal/mol respectively.  They are the unhappiest of rings – constrained into uncomfortable angles, with hydrogens forced by geometry to grumpily line up side-by-side with their repulsive neighbours. The situation for cyclopentane (ring strain: 6 kcal/mol)…